Nanoimprint technology is a development advanced from embossing technology well known in the art of optical disc production, which comprises pressing a mold original with an embossed pattern formed on its surface (this is generally referred to as “mold”, “stamper” or “template”) against a resin to thereby accurately transfer the micropattern onto the resin through mechanical deformation of the resin. In this, when a mold is once prepared, then microstructures such as nanostructures can be repeatedly molded, and therefore, this is economical, and in addition, harmful wastes and discharges from this nanotechnology are reduced. Accordingly these days, this is expected to be applicable to various technical fields.
Two methods of nanoimprint technology have been proposed; one is a thermal nanoimprint method using a thermoplastic resin as the material to be worked (for example, non-patent literature 1), and the other is a photonanoimprint method using a photocurable composition (for example, non-patent literature 2). In the thermal nanoimprint method, a mold is pressed against a polymer resin heated up to a temperature not lower than the glass transition temperature thereof, then the resin is cooled and thereafter released from the mold to thereby transfer the microstructure of the mold onto the resin on a substrate. The method has been expected to be applied in various fields, by virtue of its applicability to a wide variety of resin materials and glass materials. For example, Patent Literatures 1 and 2 discloses nanoimprinting processes capable of forming nanopatterns using thermoplastic resins at low costs.
On the other hand, according to photo-nanoimprinting process by which a nano-photocurable composition for imprints is illuminated by light through a translucent mold or a translucent substrate, so as to cure a nano-photocurable composition for imprints, it is no longer necessary to heat the material onto which a pattern is transferred when stamped under a mold, and this enables imprint at room temperature. In recent years, reports have been also issued regarding new trends of development including nanocasting process which combines advantages of the both, and reversal imprinting process capable of forming a three-dimensional stacked structure.
Applications listed below have been proposed for the imprinting.
A first application relates to that a geometry (pattern) per se obtained by molding is functionalized so as to be used as a nano-technology component, or a structural member. Examples of which include a variety of micro- or nano-optical component, high-density recording medium, optical film, and structural member of flat panel display.
A second application relates to building-up of a laminated structure by using a mold capable of simultaneously forming a micro-structure and a nano-structure, or by simple alignment between layers, and use of the laminated structure for manufacturing μ-TAS (Micro-Total Analysis System) or biochip.
A third application relates to use of the thus-formed pattern as a mask through which a substrate is worked typically by etching. By virtue of precise alignment and a large degree of integration, this technique can replace the conventional lithographic technique in manufacturing of high-density semiconductor integrated circuit, transistors in liquid crystal display device, and magnetic material for composing next-generation hard disk called patterned medium. Approaches for implementing the imprinting in these applications have been becoming more active in recent years.
As an exemplary application of the nanoimprinting process, first to be explained is an application to manufacture of high-density semiconductor integrated circuits. The semiconductor integrated circuits have been acceleratingly shrunk and integrated in these years, and for the purpose of implementing this sort of fine processing, photolithographic apparatuses used for pattern transfer have ceaselessly been improved in the preciseness. In order to address further requirements for shrinkage, it has however been becoming difficult to concurrently satisfy three requirements on resolution power of fine pattern, cost of apparatus, and throughput. In contrast, nanoimprint lithography (photo-nanoimprinting) has been proposed as a technique of forming fine patterns at low costs. For example, Patent Literature 1 and Patent Literature 3 listed below have disclosed nanoimprinting techniques using a silicon wafer as a stamper to form, by transfer, a fine structure of 25 nm or narrower. In this sort of application, a patternability as fine enough as several tens nanometers, and a high etching resistance enough to function as a mask during substrate working are required.
Paragraphs below will explain exemplary applications of the nanoimprinting to manufacturing of next-generation hard disc drive (HDD).
HDD has gone through a history of increase in capacity and down-sizing, supported by two major events, that is, advancements in performance of head and media. From the viewpoint of advancement in performance of media, the HDD has successfully increased its capacity through increase in surface recording density. Increase in the surface recording density, however, encounters an obstacle of so-called spread magnetic field from the side faces of the magnetic head. Since the spread magnetic field cannot be reduced beyond a certain level even if the head is shrunk, so that an event called “side light” will occur as a consequence. Once the side light should occur, also adjacent tracks would be written in the recording process, and thereby already-written data would be erased. Another possible event is that, due to the spread magnetic field, unnecessary signals would be read together from the adjacent tracks. There have, therefore, been proposed techniques called discrete track media and bit patterned media, which are directed to address these problems by filling a space between the tracks with a non-magnetic material so as to physically and magnetically isolate them. There have also been proposed use of nanoimprinting, as a method of forming a pattern of magnetic material or non-magnetic material in the manufacture of these media. Also in this application, required are pattern formability of several tens of nanometer level and high etching resistance enough to allow the pattern to serve as a mask.
Next, applications of the nanoimprinting process to flat displays such as liquid crystal display (LCD) and plasma display (PDP) will be explained.
With recent trends in dimensional expansion and higher definition of LCD substrate and PDP substrate, photo-nanoimprinting has recently attracted public attention as an inexpensive lithographic technique substitutive to the conventional photolithography having been used for manufacturing thin film transistor (TFT) and electrode sheet. The situation has raised a need for development of a photo-curable resist substitutive for etching photoresists having been used for the conventional photolithographic process.
There is also an ongoing trend in application of photo-nanoimprinting process to manufacture of structural members of LCD and so forth, such as translucent protective films described in Patent Literature 4 and Patent Literature 5, and a spacer described in Patent Literature 5. The resist for configuring these structural members is occasionally referred to as “permanent resist” or “permanent film”, since it finally remains in the display.
Also a spacer, which determines a cell gap of liquid crystal display device, is a sort of permanent film. In the conventional photolithography, a photo-curable composition composed of a resin, photo-polymerizable monomer and an initiator has generally and widely been used (see Patent Literature 6, for example). The spacer is formed, generally by coating a photo-curable composition over a color filter substrate having a color filter formed thereon or a color filter protective film formed thereon, followed by patterning by photolithography to a size of 10 μm to 20 μm or around, and by post-baking for curing.
Also an antireflective structure composed of a micro-pattern with a 50 to 300 nm pitch is getting noticed. Nanoimprinting is also useful for forming such pattern. This sort of antireflective structure is represented by a micro dot pattern called “moth eye”, which is effective to provide antireflection of surface of display, high-efficiency use of light by solar cell, and improved efficiency of extraction of light from LED and organic EL element. Since the thus formed patterns in these applications often remain in the final products, and are arranged in the outermost portions of articles, so that they are required to have good performances mainly in terms of durability and strength of film, including heat resistance, light resistance, solvent resistance, anti-scratching property, high mechanical performances under external pressure, and hardness.
The nanoimprinting lithography is also useful for applications regarding formation of permanent films, which include micro electro-mechanical system (MEMS), sensor device, optical components including grating and relief hologram, nanodevice, optical device, optical film and polarizing element used for manufacturing flat panel display, thin film transistor used for liquid crystal display device, organic transistor, color filter, overcoat layer, pillar component, rib member for aligning liquid crystal, micro-lens array, immunoassay chip, DNA separation chip, micro-reactor, biological nanodevice, optical waveguide, optical filter, and photonic liquid crystal.
In these applications regarding the permanent films, the thus-formed permanent films finally remain in the products, and are therefore required to be excellent mainly regarding durability and strength of the film, including heat resistance, light resistance, solvent resistance, scratch resistance, high mechanical performance under externally applied pressure, and hardness.
As described above, most of the patterns having been formed by the conventional photolithographic process may be formed by nanoimprinting, which has attracted public attention for its possibility of forming fine patterns at low costs, as known by Patent Literatures 7, for example.